A message appeared on some targeted phones asking for "$100 USD/EUR" to be sent to a PayPal account.

Networks advised affected users to contact Apple, which denied its cloud storage service had been breached.

"Apple takes security very seriously and iCloud was not compromised during this incident. Impacted users should change their Apple ID password as soon as possible and avoid using the same user name and password for multiple services," the firm said in a statement to The Register news site.

"Any users who need additional help can contact AppleCare or visit their local Apple Retail Store."

PayPal has said any funds sent to the specified account would be refunded.